1. Field of the Invention
Embodiments of the present disclosure relate to semiconductor devices and, in particular, to zinc oxide-based light emitting diodes (LEDs) and laser diodes.
2. Description of the Related Art
Interest in the development of ultraviolet semiconductor lasers for applications in photonics, information processing, and biology has increased tremendously. Zinc oxide (ZnO) has received considerable attention as a promising material for optoelectronic devices such as blue light emitting and short-wavelength laser diodes with low thresholds in the ultraviolet (UV) region. ZnO is a wide/direct band gap semiconductor, with a band gap of approximately 3.37 eV, and a large exciton binding energy of about 60 meV. These characteristics make ZnO suitable for room-temperature, exciton-enhanced lasing actions, and ZnO lasing has been achieved by optically pumping disordered particles (random lasers), nanowires (nanowire lasers), hexagonal nanonails (whispering gallery mode lasers), and thin films. Unfortunately, optically pumped lasers require even shorter wave-length lasers as pumping sources, resulting in limited potential toward application.
Electrically pumped ultraviolet ZnO semiconductor diode lasers are more suitable for practical applications but few articles have been published about ZnO lasers in the literature. Rather, the literature on green/blue lasers reports on gallium nitride (GaN)-based laser diodes with edge emitting geometries, where the laser light propagates parallel to the wafer surface of the semiconductor.
Such scarcity, even after about a decade of worldwide intensive research efforts an ZnO thin films and nanostructures, may be attributed, at least in part, to a lack of reliable and controllable p-type ZnO. The fabrication of p-type ZnO is relatively difficult due to the self-compensating effect from native defects (Vo and Zni) and/or Hydrogen (H) incorporation. Moreover, the low solubility and the deep acceptor levels of the dopants may yield low carrier concentrations, making p-type ZnO even harder to fabricate.